The role of negative pressure wound therapy with instillation and dwell time in the treatment of deep sternal wound infections—A retrospective cohort study

Abstract Background and Aims Negative pressure wound therapy (NPWT) has gained a central role in the treatment of deep sternal wound infections (DSWIs) after median thoracotomy. Our study aims at proving the safety of using NPWT with instillation and dwell time (NPWTi‐d) in the treatment of DSWI. Methods We retrospectively evaluated the patients who were treated at our institution between March 2018 and November 2021 for DSWI after radical sternectomy using NPWT or NPWTi‐d. The NPWTi‐d was applied to start the first postoperative day using 75 mmHg negative pressure for 3 h, followed by instillation of sodium hypochlorite <0.08% with a 3‐min dwell time. Results The NPWTi‐d group showed a shorter length of stay (29.39 ± 12.09 vs. 39.54 ± 17.07 days; p = 0.049), a shorter elapsed time between the debridement and the flap coverage (7.18 ± 4.27 vs. 11.86 ± 7.7 days; p = 0.003) and less operative or nonoperative dressing changes (1.73 ± 1.14 vs. 2.68 ± 56; p < 0.001). The in‐hospital mortality was 8.2%, with no significant differences between the two groups (p = 1). Conclusion NPWTi‐d can be safely employed in the treatment of DSWI. Further prospective randomized studies need to establish the role of NPWTi‐d in the control of infection and biofilm as well as in wound healing.

The evolution of negative pressure wound therapy (NPWT) has radically changed the treatment course and outcomes of patients with DSWI, decreasing the in-hospital mortality from 36% to 52% 3 before the NPWT era to around 10% nowadays. 4 NPWT ensures the drainage of secretions, promotes vascularity and granulation while sealing the wound. It can be used either as a bridge until flap closure or as vacuum-assisted wound closure. 5,6 The further development of NPWT has led to the addition of instillation and dwell time (NWPTi-d). Since its commercial introduction in 2003, NPWTi-d has found a wide range of applications, including pressure sores, burns, traumatic wounds, diabetic and venous ulcers, or open amputations. The evidence shows that in these types of wounds NPWTi-d causes a reduction in hospital stay, time to wound closure, number of surgical debridements and dressing changes. 7 Furthermore, NPWTi-d has been proven to accelerate the granulation by up to 43% and reduce the bioburden by up to 48%. 8 The technique involves the instillation of a solution with a recommended dwell time of 10-20 min and a negative pressure time of 2-4 h at −125 mmHg. There is no consensus concerning the instillation solutions, which can be saline, hypochlorite, biguanide, or iodine solutions. 9 Although the features of NPWTi-d could accelerate infection control in DSWI, there is little data in the literature about its usage in the treatment of DSWI and the present guidelines for NPWTi-d do not mention these wounds. 9,10 Our study aimed to evaluate the safety of NPWTi-d in the treatment of DSWI. Secondarily, we aimed to identify the possible benefits of NPWTi-d as opposed to NPWT in treating these patients.

| Therapeutical procedure
After the diagnosis of DSWI (Figure 1), a radical sternal debridement is performed with radical sternectomy (Figures 2 and 3). If the pleura parietalis is opened during the debridement, the opening is closed either by suture or by a fibrin sealant patch. If, due to adhesions, the lung parenchyma is opened, a similar procedure is performed. In cases where the pericardium is opened after the debridement, closure is tempted using a direct suture. If this is not possible, NPWTi-d is contraindicated due to the theoretical risk of pericardial tamponade.
Finally, if bleeding control is achieved, foam is used to obliterate the mediastinal wound, and then the adhesive drape is used to seal the wound. A trackpad with two tubes is applied. One tube is connected to the canister tubing and the NPWT device is set to 75 mmHg. A higher pressure has been proven in our experience to increase the pain and discomfort of the patient. The second tube is firstly clamped and will be later used for instillation. To avoid washing out the clots and causing postoperative bleeding, the NPWTi-d is started on the first postoperative day. At this moment, the second tube is declamped and connected to the instillation device, using a sodium hypochlorite solution <0.08%. The instilled quantity is estimated at the first application so that the solution will not spill out of the wound and detach the adhesive drape. Usually, 30-50 mL is necessary for the mediastinal wound. The negative pressure therapy time is set for three hours, with a dwell time of three minutes. This ensures that antimicrobial action takes place while minimizing the risk of drape detachment with loss of vacuum effect.
If, due to anticoagulation and anti-aggregation, there is diffuse bleeding after the debridement, which cannot be controlled, the wound is packed with cotton gauze and sealed with adhesive tape. The NPWT dressing is then applied on the first postoperative day and the instillation  Table 1).
When evaluating the intraoperative incidents, we found no significant difference between the two groups considering the opening of the parietal pleura, the lung parenchyma, or the pericardium ( Table 2).  Furthermore, the number of surgeries or bedside dressing changes performed until flap closure was significantly less in the NPWTi-d group (1.73 ± 1.14 vs. 2.68 ± 1.56; p < 0.001, MWU; Table 3).
The CT scans before the debridement showed no pneumothorax, whereas after debridement in three cases and after flap coverage in five cases an asymptomatic pneumothorax was diagnosed, without a significant difference between the two examined groups (p = 0.58 and p = 1, FET). No pericardial tamponade was recorded. Moreover,    the thorax CT scans were evaluated for the occurrence of pleural effusion before and after the debridement as well as after the flap coverage. The findings showed no significant differences between the two examined groups in any of the examined moments (Table 4).
Finally, the in-hospital mortality was 8.2% (seven patients), with no significant differences between the two groups (4 vs. 3 patients, p = 1, FET).

| DISCUSSION
In the present study, we compared the complications and early procedure has also greatly reduced the need for bedside dressing changes, which classically lead to pain, further wound contamination, and possible major organ injury. Although some authors propose to use of NPWT as a means of wound closure, 6 we believe that the swift and effective flap closure ensures better results with less stress to the patient and better survival rates. 5 One major risk to the employment of NPWT in DSWI is the risk of organ rupture by adhesion and ingrowth of the tissue into the foam. While the lungs can be relatively easily resealed as described before and the great vessels are seldomly directly exposed, there are several reports of bypass vessel or right ventricular rupture with NPWT dressing change with catastrophic consequences. 12,13 This complication may occur when the pericardium has been partially removed and the myocardium is directly exposed. In these cases, we employ for protection a perforated silicone sheath between the exposed organ and the foam. For the utilization of NPWTi-d, guidelines have been issued, which recommend its use in different types of chronic wounds, including the use with caution in wounds that contain appropriately protected organs or vessels. 15 Although the DSWI were not specifically described so far in these guidelines, one could allocate these wounds in the latter description.
T A B L E 3 Treatment outcomes.  Considering all these, we find that instillation with NaOCl with a low concentration (0.08%) for 3 min every 3 h may provide an optimal antimicrobial effect while minimizing the toxicity and the risk of seal compromise. Nevertheless, a 5-min dwell time can be taken into consideration, to maximize the antimicrobial effect. The recommended negative pressure of −125 mmHg causes in our experience pain and discomfort, and therefore, we employ 75 mmHg, which is better tolerated by the patients.

ACKNOWLEDGMENTS
Konrad Schubert advised on the statistical analysis of the data. Open Access funding enabled and organized by Projekt DEAL.

CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
The data sets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

ETHICS STATEMENT
Institutional review board (IRB) approval from the Ethics Committee of TU Dresden was obtained (BO-EK-387082020). An informed consent to participate was waived by the Ethics Committee of TU Dresden. All methods were carried out in accordance with the guidelines and regulations.

TRANSPARENCY STATEMENT
The lead author Olimpiu Bota affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.